Two-by-two optical directional couplers are used to switch guided-wave optical signals. A problem with these guided-wave directional couplers is the loss of power associated with coupling in and out of them. As a result, the size of a switching matrix containing these directional couplers is limited. One such directional coupler is the lithium niobate crossbar coupler which has an elongated physical structure. As a consequence, any sizable switching matrix containing this crossbar coupler would have to be physically folded which would introduce additional power losses.
Optical cross-bar switches for switching an unguided optical signal in free-space from any position in a single column to one selected position in a single row are also known. A two dimensional array of selectors/decoders is positioned typically between two lenses to implement this crossbar optical switch. However, these optical switches are limited to switching optical signals in only one dimension, i.e., a single column to a single row or vice versa.
A single stage of these prior art optical crossbar switches can perform time-division optical switching such as rearranging serial bits received on an optical fiber. A single stage of these optical switches can also perform space-division switching in which a serial bit stream is switched form one input fiber to a selected output fiber. Furthermore, several stages of these optical switches may be interconnected to perform space- and time-division switching. However, the problem is that these optical crossbar switches can not be combined to perform both space- and time-division optical switching with only a single switching stage. As previously suggested, these optical crossbar switches are also limited to switching optical signals in one dimension.